YUGOSLAVIA BEEFS UP FORCES IN KOSOVO CAMPAIGN.

Byline: Steven Lee Myers The New York Times

Despite four weeks of NATO air and missile strikes, Yugoslavia's army and police forces have sent thousands of reinforcements into Kosovo and stepped up the use of helicopters and aircraft to press their campaign against the province's ethnic Albanians, NATO and Pentagon officials said Monday.

The reports of reinforcements - and heavy fighting in several parts of Kosovo - appeared to undercut NATO's assertions that it is choking off the ability of Yugoslavia to resupply its forces in the field. The officials said they based their reports on intelligence information.

The inability to stop the Yugoslav offensive, after more than 6,000 allied sorties, has forced NATO to consider ways to expand the scope and intensity of its attacks.

In a development that could open up a new avenue for the alliance's airplanes to attack Serbia from the east, Bulgaria agreed Monday to allow NATO to freely cross its airspace.

The United States and NATO have asked several Balkan states to consider allowing overflight or providing bases to accommodate more than 300 additional aircraft that the alliance's supreme commander, Gen. Wesley K. Clark, has requested for the strikes.

Over the weekend, President Clinton spoke to Bulgarian President Petar Stoyanov, along with leaders of Romania, Albania and Hungary, and discussed contributions their countries could make, a senior administration official said. The official declined to elaborate on options discussed by the nations.

``We're trying to get a number of things lined up,'' the official said.

NATO officials also are considering basing aircraft in Hungary, one of the alliance's three new members, and could consider putting additional aircraft in Turkey to use the Bulgarian airspace. Another option being considered is using the aircraft carrier Enterprise to broaden the air attack, which would mean devoting two of the nation's 12 aircraft carriers to the conflict.

A day after reporting one of the most intense nights of bombing since the air assault began March 24, NATO officials said Monday that rain overnight forced the cancellation of a majority of raids. Even so, they reported strikes against oil complexes at Novi Sad and Smederevo and ammunition storage or manufacturing sites in Paracin, Pristina and Bujanovac.

The attacks continued for a 27th day Monday, although with less intensity.

Troop estimates

At NATO's headquarters in Brussels, officials continued to point to the successes of the strikes, including the disruption of Yugoslavia's oil supply, but so far the attacks have done little to cripple the campaign. NATO spokesman Jamie Shea disclosed that ``about 8,000 extra Yugoslav forces'' had been sent to Kosovo.

In Washington, Pentagon spokesman Kenneth Bacon put the increase at about 3,000, saying the total number of Yugoslav army troops and paramilitary police in the province could be as high as 43,000. Officials later said the increase - which was based on intelligence reports and satellite photographs - had occurred gradually as the Yugoslav army and police shored up units carrying out a sweep of villages in Kosovo.

Shea said the reinforcements underscored Yugoslavia's difficulties in quashing rebels from the Kosovo Liberation Army, whose fight for independence for the province prompted the Yugoslav crackdown that has led to NATO's bombing campaign.

``I think it's a sign of how difficult Belgrade is finding it to bring the area under total control,'' he said.

At the same time, the buildup showed that the Serbs who dominate Yugoslavia's military are still able to fight, despite an intensification of NATO's bombing to include more daylight strikes against ground forces in Kosovo.

NATO's military spokesman, Brig. Gen. Giuseppe Marani of Italy, reported an increase in attacks by Yugoslav planes and helicopters, including many Sunday. NATO aircraft, which commanders have said are able to strike at will over Yugoslavia, were not close enough to them to attack, he said.

Marani dismissed the significance of Yugoslavia's air force, which has been repeatedly targeted by NATO warplanes and missiles. ``If they succeed in taking off, their mission has to be short, has to be of limited size and with limited scope,'' he said.

In recent days there have been new reports of clashes, not just along Kosovo's border with Albania, where the rebels are strongest, but also in other parts of the province, including Mitroveic, Podujevo and Malisevo. ``In fact, we have reports of large movements of Serb forces across Kosovo as these operations continue,'' Shea said.

Shea said the Serbs were driving about 150,000 of Kosovo's ethnic Albanians from the north toward the provincial capital, Pristina, in what he called ``a kind of anti-humanitarian corridor.''

``This suggests that this is not simply random ethnic cleansing, but it's being done on an almost scientific and systematic basis,'' he said.

However, NATO and Pentagon officials remain puzzled by the Yugoslav strategy. On Sunday, the officials said Monday, some 40,000 refugees crossed into Macedonia and Albania. By Monday, however, only a trickle did as Yugoslav authorities again reportedly closed its borders.

Clark and other commanders have been frustrated by NATO's inability to reverse Yugoslavia's success in driving hundreds of thousands of ethnic Albanians from the province.

More than a week after Clark asked for 300 additional aircraft, bringing the total NATO force to more than 1,000, the Pentagon has still not formally signed off on the request. Officials said approval was expected, but because the request will require Clinton to authorize the activation of as many 33,000 reservists and guard members, the process has moved sluggishly.

Finding bases

Another issue has been finding bases for the aircraft. Italy's facilities, which have been the fulcrum of the strikes, already are overloaded. Officials said they expected to be able to announce the deployment of the additional aircraft later in the week, once they had worked through the sensitivities in the region.

The deployment of two dozen Apache helicopters to Albania - part of the effort to strike at the Yugoslav forces directly involved in the killings - also has suffered new setbacks.

The first of the Apaches, now in Italy, were supposed to arrive in Albania's capital over the weekend, but were grounded by weather.

At the Pentagon on Monday, a spokesman tried to explain the latest delay by showing photographs of deep mud at the airfield where the Apaches were supposed to land. For reasons that were not clear, the American forces sent in advance of the helicopters made their base in a low-lying field near the airport.

``So the mud in the sector has accumulated,'' said Maj. Gen. Charles F. Wald, the deputy director of strategic plans and policy for the Joint Chiefs of Staff. ``And this is part of the reason it is taking a little time to get there: Mother Nature.''

He said the Apaches might arrive today, but are not expected to be ready for combat until next week, more than a month after Clark first requested them.

Impatience over NATO's strategy continues to reverberate in Washington. Clinton sent a request to Congress on Monday for more than $6 billion in emergency funds, most of it to pay for the Pentagon's aircraft, ships, missiles and bombs in the Kosovo conflict. The request might well ignite a heated debate over the state of the American military, its role in the Balkans, and the readiness of its arsenal.

Clinton called on Congress to approve the money quickly, without rancor. ``The need for this funding is urgent, immediate, clearly in the national interest,'' he said. ``There are literally lives hanging in the balance.''

KOSOVO DEVELOPMENTS

COMBAT

On the 27th day of the air assault on Yugoslavia, NATO struck a communications transmitter near the Kosovo capital, Pristina, and an airport near Urosevac in southern Kosovo. During the preceding night, NATO planes bombed Pristina and Novi Sad, Serbia's second-largest city.

NATO acknowledged for the first time that its aircraft had hit two separate convoys in southwestern Kosovo last week and might have killed civilians.

Separatist rebels of the Kosovo Liberation Army said they had captured three members of the Yugoslav army.

REFUGEES

The flow of Kosovar Albanians into Albania and Macedonia dried up, raising fears among aid workers for the safety of tens of thousands of refugees. The U.N. high commissioner for refugees said Yugoslav forces appeared to be turning back Albanians trying to leave the country.

The U.N. refugee agency estimated that more than 619,000 refugees had fled to neighboring countries since conflict broke out in Kosovo more than a year ago.

NATO was considering parachuting relief aid to displaced Albanians within Kosovo.

DIPLOMACY

President Clinton called President Boris Yeltsin of Russia to press for Russian involvement in a resolution that would involve Russian troops in an international force in Kosovo. It was their first conversation on the conflict since the bombing began March 24.

U.N. Secretary-General Kofi Annan said he would visit Moscow on April 29 to discuss the Kosovo crisis.

OTHER DEVELOPMENTS

- The Clinton administration asked Congress for $6 billion in emergency money for the Kosovo conflict.

In the Yugoslav republic of Montenegro, residents of three Muslim villages near the border with Kosovo said the Yugoslav army had expelled them to turn their communities into a second front line.

CAPTION(S):

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Box: Kosovo developments (see text)